Today, the heat generated by dense electronic devices is an expensive resource drain. To keep systems at the right temperature for optimal computing performance, data center cooling systems in the United States consume as much energy and water as all the residents of Philadelphia. Now, by integrating liquid cooling channels directly into semiconductor chips, researchers hope to reduce this drain, at least in power electronic devices, making them smaller, cheaper, and more energy-efficient.
Traditionally, electronic devices and thermal management systems have been designed and manufactured separately, says Elison Matioli, professor of electrical engineering at the Swiss Federal Institute of Technology in Lausanne (EPFL). This presents a fundamental obstacle to improving cooling efficiency, as heat must travel a relatively long distance through multiple materials to be removed. For example, in today's processors, thermal siphons transfer heat from the chip to bulky air-cooled copper heat sinks.
To achieve a more energy-efficient solution, Matioli and his colleagues developed a low-cost process that places a 3D network of microfluidic cooling channels directly into the semiconductor chip. Since liquids remove heat better than air, the idea is to keep the coolant micrometers away from hot spots on the chip.
But unlike previously reported microfluidic cooling technologies, he said, "We designed the electronics and cooling system from the very beginning." Therefore, the microchannels are located beneath the active region of each transistor device, where the temperature is highest, which improves cooling performance by 50 times. They recently reported their shared design concept in the journal *Nature*.
Researchers first proposed microchannel cooling technology as early as 1981, and startups like Cooligy have been pursuing the concept in processors. However, the semiconductor industry is shifting from planar devices to three-dimensional devices and moving towards future chips with multi-layered structures, making cooling channels impractical. “This embedded cooling solution doesn’t work for modern processors and chips like CPUs,” says Tiwei Wei, who researches electronic cooling solutions at the Interuniversity Microelectronics Center and KU Luuven in Belgium. “Instead, this cooling technology makes the most sense for power electronics,” he says.
Power electronic circuits manage and convert electrical energy, and are widely used in computers, data centers, solar panels, and electric vehicles. They utilize large-area discrete devices made of wide-bandgap semiconductors such as gallium nitride. The power density of these devices has increased dramatically in the past few years, meaning they must be “hung up with a huge heatsink,” Matoli said.
Recently, power electronic modules have shifted towards liquid cooling, whether through cold plates or microchannel cooling systems. However, to date, all microchannel cooling systems have been fabricated separately and then bonded to the chip. Bonding layers increase heat resistance, but channels and circuitry are not tightly aligned.
“We’ve taken it to the next level,” Matoli said, by creating both the device and cooling channels within the same chip. They etched micrometer-wide cracks in a gallium nitride layer coated on a silicon substrate. The cracks were 30 μm long and 115 μm deep. Using a special gas etching technique, they widened the cracks in the silicon substrate, creating channels for the liquid coolant to pass through.
The researchers then used copper to seal tiny openings in the gallium nitride layer to fabricate devices on top of it. He said, "We only have microchannels in tiny areas of the wafer, and these microchannels are in contact with each transistor. This makes the technology much more efficient because we can extract a lot of heat from nearby, but the pumping power we use is very small."
As a demonstration, the researchers fabricated an AC-DC rectifier circuit consisting of four Schottky diodes, each capable of handling 1.2kV. Circuits like this typically require a heatsink the size of a fist. However, the circuit chip with an integrated liquid cooling system was mounted on a three-layer printed circuit board the size of a USB flash drive, with channels etched onto it to deliver coolant to the chip.
The results show that hotspots with power densities exceeding 1700 watts per square centimeter can be cooled using only 0.57 watts per square centimeter of pumping power. This represents a 50-fold performance improvement compared to previously reported microfluidic channel cooling.
Wei said, "The reliability of the gallium nitride thin film and copper sealing layer should be studied over time. But this innovative cooling solution is a big step towards a low-cost, ultra-compact and energy-efficient power electronics cooling system."
